The present invention relates to a method and apparatus for reducing the power consumption of a mobile station in a cellular radio system.
Known cellular radio systems include a plurality of base stations for providing service in predetermined geographical areas, or cells. Mobile stations situated in the cells are serviced by one or more base stations covering that particular cell.
A mobile station will typically operate in one of two modes depending on the needs of the user. When the user is making a telephone call the mobile station is said to be in dedicated mode in which the mobile station is actively transmitting and receiving on a channel allocated for its own use. However at any other time when the mobile station is powered up the mobile station is said to be in idle mode and may be contacted by other users. In idle mode the mobile station is still in contact with the base station but there is no dedicated channel for sending and receiving speech or data. Contact is achieved with the base station through the mobile station listening to broadcast channels which constantly update the mobile stations within the cell with necessary information.
Methods of transmitting and receiving speech and data in dedicated mode are known in the art and are detailed with reference to the GSM (Groupe Speciale Mobile) system in the publication M. R. L. Hodges "The GSM radio interface, British Telecom Technological Journal", Vol. 8, No 1, 1990, p. 31-43, section 4, the content of which is hereby incorporated by reference.
For handheld mobile stations which are battery powered the maximum time in which the mobile station can operate in idle mode is typically about 16 hours.
For a similar mobile station in dedicated mode the maximum operational time is typically about 1 to 2 hours. These periods of operation are dependent almost entirely on the power consumption of the mobile station in the respective modes. A reduction of the power consumption in either mode would result in increased operational times. In idle mode the main contributor to the power consumption typically comes from the RF (Radio Frequency) sections of the mobile station. Therefore to reduce power consumption it is favorable for the RF section to only be on for limited periods of time. As the main purpose of the RF section in idle mode is to listen to broadcast messages it is further noted that the receiving side of the RF section is the most significant.
Generally broadcast messages sent by a base station which are specific to one mobile station will normally be received by all the mobile stations in idle mode within the cell. This obviously has an adverse effect on the operational time of these mobile stations and to this avail European patent publication EP 473465 suggests that for messages containing an identifying part and an information part, the mobile station first receives and decodes the identifying part. If the mobile station then discovers that the message is intended for itself the information part is further received and decoded. However, if the mobile station discovers that the message is intended for another mobile station the receiving circuits are switched to power saving mode during reception of the information part. Battery saving according to EP 473 465 is therefore based on a method for receiving two-word message, the first word of which contains the address. If this address indicates that the message is intended for another mobile station then, according to EP 473 465, it is not necessary to receive the second word of the message. Thus it is possible for the mobile station to switch a considerable part of its receiving circuits into the power saving mode until the next message is expected to arrive. This power saving mode is controlled by a timing circuit, which may be programmed with the time intervals between messages. The receiving of a message according to EP 473 465 is discontinued on the basis of receiving an address part of a complete message.
The prior art will now be discussed further with reference to the GSM system of communication. However, it should be noted that the invention is not restricted purely to this system but that reference to the GSM system aids a clear description and understanding of the state of the art for the present invention.
The idle mode and the dedicated mode in the GSM system have two classes of channels associated with them, notably dedicated channels for dedicated mode and common channels for idle mode. Referring to the common channels used in idle mode the most frequently used are the downlink channels, i,e. the channels for sending information from the base station to the mobile station. These are divided still further into channels for control information such as the FCCH (Frequency Correction CHannel) and the SCH (Synchronisation CHannel) and those for sending messages, the BCCH (Broadcast Control CHannel) and PCH (Paging CHannel). It is these downlink channels the mobile station listens to and they represent the main power consumption when in idle mode. The form of these channels are shown in FIG. 3.
The BCCH and PCH are similar in that they are encoded in exactly the same way and are sent over the same number of TDMA (Time Division Multiple Access) frames. However, the actual messages they convey differ substantially. The BCCH is used for sending messages to all mobile stations, relating to, for example, cell selection, network identification, as well as others. The PCH is used for paging specific mobile stations but is nevertheless monitored by all mobile stations within the cell. To describe how these messages are received it is necessary to first understand the basics of how these messages are transmitted.
Firstly the PCH or BCCH message originates in digital form from the infrastructure of the GSM system. From there channel coding is performed on the data which introduces redundancy into the message by adding extra information relating to the original data. This allows messages to be reconstructed even when information is lost over the radio link. The channel encoded message is then interleaved to mix up the data within the message in a predetermined way. Following this the message is then encrypted, formatted for burst formation, modulated, then transmitted. The resulting message is transmitted in four transmission bursts (timeslots) in four consecutive TDMA frames.
Mobile stations in idle mode receive all the four timeslots corresponding to the complete message. Then the message is reconstructed according to the inverse operations of the transmission. The process involves receiving the signal from the antenna, demodulating the signal, de-encrypting the message, de-interleaving the message, then channel de-coding the message. The result is the original message sent from the base station.